:S ive been under the impression that with hplc, the stationary phase was the packing inside of the column (eg silica) and then the solvent was the mobile phase. Then there's a pump attached to the front of the column that constantly pumps more of the solvent/mobile phase through the column, sweeping the sample along and with it and separating out the components.
in normal phase chromatography, the stationary phase is more polar than the mobile phase and so components that are highly polar have a higher affinity to the stationary phase than a component that is less polar. hence, polar components spend more time adsorbed onto the stationary phase and so would stay in the column for longer/have a higher retention time than non polar components. in reverse phase chromatography, the stationary phase is less polar than the mobile phase so the opposite occurs; polar components are eluted from the column faster than non polar components and have a lower retention time.
EFPBH, you pretty much need to work out the mole for any chemistry question. to find the empirical formula, you're trying to find the atom ratio of each element in the hydrocarbon. moles are just a convenient way of counting atoms.